The CAST UDL Book Builder is a "free online tool (that) enables educators to develop their own digital books to support reading instruction for children aged 3 and up. Teachers create, edit, and save resource-rich texts."
When I clicked through to the Book Builder I found a collection of multimedia resources that enable designers and instructors to embed music, audio recordings of explanations or discussions, hyperlinks, and more into the lesson books they create.
One of my favorite resources here is the student response area, which encourages active learning and meaningful interaction with educational and instructional texts.
For me, the “wow” factor is that the site and its resources facilitate multimedia instruction for all learners, including those who do not or cannot interact with visual media.